I understand why they are spaced relatively evenly. But couldn't it just as practically be the following?...

20mm32mm47mm80mm

Ultimately, it seems like the "standard" lengths are probably arbitrary in their origins. Probably the first person to pick the original standard focal length was making a technical decision, rather than an aesthetic decision.

I've often wondered the same thing. I have 28, 50, 85, 135, and 200 primes. Macros tend to be 50 and 100 though. I noticed the trend of going up by 1.6, roughly the ratio of crop to full frame, so it was easy to think "just go one focal length up on a full frame from what I used on a crop". Why go up by 1.6 rather than, say, the square root of 2?

Actually, it would (kind of) make sense if every next FL had around 2x smaller FoV, like:12mm<17mm<24mm<35mm<50mm<70mm<100mm<135mm<200mm<300mm<400mm...or10mm<14mm<20mm<28mm<40mm<60mm<85mm<120mm<180mm<250mm<350mm<500mm...

If you compare fast Canon L primes24/14~1.71 >> 1.71*1.71~2.92x35/24~1.46 >> 1.46*1.46~2.13x50/35~1.43 >> 1.43*1.43~2.04x85/50=1.7 >> 1.7*1.7=2.89x135/85~1.59 >> 1.59*1.59~2.53x200/135~1.48 >> 1.48*1.48~2.19xthen you'll see that they are not evenly spaced. The FoV difference varies from around 2x to almost 3x.

50mm is the single easiest focal length to design for a 35mm camera and it's a very nice even, basic number so that probably explains that. For 35 and 85 they probably just picked a nice div 5 number close to focal lengths that seemed to be both quite useful for certain purposes and far enough away from 50mm. Why 24mm became much more the standard than 25mm I have no idea though.

I cannot say for sure but my guess is when the first lenses were being made in the 1800s for medium and large format cameras the photographer scientists manufactured lenses with whatever focal lengths their state of the art technology allowed them to be made. Then those focal lengths became standardized because others also made similar lenses. Each of these focal lengths gave rise to a particular angle of view. Then probably when the 35 mm camera came the lens makers tried to emulate those angle of views and some particular focal lengths became manufactured repeatedly and became popular and then standardized. But all these are my guesses.

I guess it all centres around the 'normal' 50mm lens, even though 43mm is a better fit for a 35mm frame.

As for the actual focal lengths and apertures of lenses, remember the specs quoted are usually a near fit for the actual real life specs of the lens (as is often revealed in the patents), so they tend to get rounded to the nearest fit.

I've heard, in part, that the 45-55 range is approximately equal to the field of view of the human eye and so that formed the basis for the 'normal' lens being about 50mm, and wideangle at ~35mm is because, depending on the exact camera, that's only slightly shorter than most flange depths, and so only needs a mild or non- retrofocus design. Also apparently somewhat by convention, https://en.wikipedia.org/wiki/Wide-angle_lens#Wide-angle_lenses_for_35_mm_format.

I've heard, in part, that the 45-55 range is approximately equal to the field of view of the human eye and so that formed the basis for the 'normal' lens being about 50mm, and wideangle at ~35mm is because, depending on the exact camera, that's only slightly shorter than most flange depths, and so only needs a mild or non- retrofocus design. Also apparently somewhat by convention, https://en.wikipedia.org/wiki/Wide-angle_lens#Wide-angle_lenses_for_35_mm_format.

According to the prosecuting attorney during the George Zimmerman trial, the human eye sees at 49.6mm. Yes, I actually learned something useful during the trial

"Normal" focal length for 35mm film is 43mm, which got rounded up to 50 by Oskar Barnack. The idea behind the progression of primes is this:

A 75mm lens will give you roughly the same framing from top to bottom when used in portrait orientation as a 50mm would when used in landscape. Put another way, if you took a photo take at 50mm in landscape orientation and cropped away the left 1/3 and right 1/3 of the frame, you'd get roughly the same image as if you used a 75mm lens in portrait orientation. Kind of awkward to describe so I hope that makes sense.

So, it's not completely arbitrary. All primes are multiples of 43mm or 50mm (1.4x, roughly the square root of 2, much like f-stops). "Normal" focal lengths for larger formats are also familiar numbers: 85mm (medium format), 200mm (large format) etc.

In my opinion, if you have a field of view of 46°, you should contact an ophthalmologist!The field of view of a 50mm lens, on a 35mm sensor, is similar to the "field of focus" of the human eye and it has a similar perspective rendition, but its field of view is much narrower.